Electromechanical transducer and method of making same

ABSTRACT

An electromechanical transducer suitable for push-pull operation and a method of making the same. The transducer utilizes stators comprising electrets and a diaphragm comprising a layer of conductive material supported between the electret stators. The stators may be electrostatically polarized after assembly with the diaphragm, providing high conversion efficiency and low distortion. The transducer is particularly adapted for use in an earphone.

United States Patent [1 1 Chang l l ELECTROMECHANICAL TRANSDUCER AND METHOD OF MAKING SAME [75] Inventor: Loh-Yi Chang, Guelph, Ontario,

Canada [73] Assignee: Uniroyal Ltd., Montreal, Canada (22] Filed: May 11,1973

[2|] Appl. No.: 359,568

{30) Foreign Application Priority Data Sept. 8, 1972 Canada l5l244 [52] US. Cl. 29/594; 179/l 11 E [5i] Int. Cl H04r 19/02; H04r 31/00 [58] Field of Search l79/lll E, 111 R; 307/88 ET; 29/594 [56] References Cited UNITED STATES PATENTS 3,389,226 6/l968 Peabody l79/lll R 'LI Z8 [4 1 July 15, 1975 3 474.l97 10/1969 Koural et al. 179/] ll E 3,646,280 2/1972 Tamura et al 179/111 E FOREIGN PATENTS OR APPLICATlONS l,l47,423 4/1969 United Kingdom 129/1! l E Primary Examiner-Kathleen H. Claffy Assistant Examiner-George G. Stellar Attorney, Agent, or Firm-Charles A. Blank [57] ABSTRACT 2 Claims, 8 Drawing Figures to e .2

34 33 lOa.

SHEET lob MTEHTFDJUL 13 1975 SHEET Jed ELECTROMECHANICAL TRANSDUCER AND METHOD OF MAKING SAME This invention relates to electromechanical transducers and to methods of making the same. More particularly, the invention relates to electromechanical transducers utilizing a polarized dielectric material such as an electret in a construction suitable for push-pull operation without requiring an external direct-current bias voltage.

One prior transducer utilizes a diaphragm comprising a number of layers of thin electret material and a num' ber of conductive layers positioned between two electrodes for push-pull operation. The dynamic mass of that diaphragm is usually higher than that of an electrostatic transducer of similar design which does not use electret materials but uses an external directcurrent bias voltage. The higher dynamic mass of the electret diaphragm adversely affects the fidelity and conversion efficiency of the transducer. Moreover, in order to obtain good fidelity, the air gap space between the diaphragm and the stationary member of the transducer should be of the order of approximately mils. have proven mathematically that the vibrational force on the diaphragm is a maximum when the thickness of the electret is greater than the air gap space, that is, when the thickness of the electret equals the dielectric constant of the electret multiplied by the air gap space. Thus, the maximum vibrational force on the prior electret diaphragm described above cannot be realized practically because the electret diaphragm would be too thick to vibrate.

Due to the relatively thicker construction of the electret of a transducer constructed in accordance with the invention the equivalent direct current bias voltage may be at least one kilovolt. This is higher than the bias voltage of conventional electrostatic earphones, which is usually approximately 200 volts. Accordingly, high efficiency and stability can be obtained in transducers constructed in accordance with the invention without the high voltage hazard of an external bias voltage necessary for conventional electrostatic earphones.

It is an object of the present invention to provide a new and improved electromechanical transducer which avoids one or more disadvantages of such prior transducers.

it is another object of the invention to provide a new and improved electromechanical transducer utilizing an electret construction and suitable for push-pull operation and providing good fidelity and good electromechanical conversion efficiency.

It is another object of the invention to provide a new and improved electromechanical transducer of low weight, for example, 0.063 pound and small thickness, for example, 200 mils.

It is another object of the invention to provide a new and improved method of making an electromechanical transducer.

in accordance with the invention, an electromechanical transducer comprises a pair of electrostatically polarized dielectric stator means each having a conductive member and each having apertures therethrough. The transducer also includes thin, vibratory diaphragm means comprising a layer of conductive material, means for supporting the diaphragm means between the stator means in spaced relation thereto, and means for individually connecting electrically to the conductive members of the stator means and the conductive layer of the diaphragm means.

Also in accordance with the invention, the method of making an electromechanical transducer comprises assembling in spaced relation a pair of dielectric stator means each having a conductive member and each having apertures therethrough and thin, vibratory diaphragm means comprising a layer of conductive material between the stator means, and electrostatically polarizing the assembled stator means by applying a high potential difference from the conductive member of each stator means to the conductive layer of the diaphragm means.

Also in accordance with the invention, an electromechanical transducer comprises electrostatically polarized dielectric stator means having a conductive member and having a dielectric constant. The transducer also includes thin, vibratory diaphragm means comprising a layer of conductive material, means for supporting the diaphragm means in spaced relation to the stator means, the thickness of the dielectric stator means exclusive of the conductive member being approximately equal to the product of the dielectric constant times the distance between the diaphragm means and the stator means, and means for individually connecting electrically to the conductive member of the stator means and the conductive layer of the diaphragm means.

By stator means is meant means which is stationary within the transducer insofar as mechanical vibrations, for example, sound-reproducing or sound-responsive vibrations, within the transducer are concerned.

For a better understanding of the present invention, together with other and further objects thereof, reference is made to the following description, taken in connection with the accompanying drawings, and its scope will be pointed out in the appended claims.

Referring now more particularly to the drawings:

H6. 1 is a plan view, partly schematic, of a transducer suitable for use in an earphone and constructed in accordance with the invention;

FIG. 2 is a fragmentary, sectional view, partly schematic, of the FIG. 1 transducer, taken along line 22 of FIG. 1, with the transducer drawn to an enlarged scale;

FIG. 3 is a plan view, partly schematic, of a stator of the FIG. I transducer;

FIG. 4 is a perspective view, partly schematic, of the diaphragms and supporting frames of the H6. 1 transducer;

FIG. 5 is a fragmentary, sectional view, partly schematic, of the H0 4 diaphragms, drawn to an enlarged scale and taken along line 55 of FIG. 4;

FIG. 6 is a plan view of a frame utilized to space the diaphragms of FIG. 4;

FIG. 7 is a fragmentary sectional view, to an enlarged scale, of the FIG. 1 transducer, taken along line 7-7 of PK 1, representing means for connecting electrically to the conductive layers of the diaphragm means; and

FIG. 8 is a diagram, partly schematic, of two earphone transducers constructed in accordance with the invention and electrical circuits connected thereto suitable for coupling to sources of electrical audiofrequency signals representing stereophonic sound.

Referring now more particularly to FIGS. 1 and 2 of the drawings, an electromechanical transducer in accordance with the invention and suitable for use in an earphone is there represented. The transducer may be housed in a conventional earphone casing (not shown) and may be attached thereto by any suitable means, for example, by contact cement.

The transducer comprises a pair of polarized dielectric stator means 10, 11 each having a conductive layer thereon and each having apertures 10a, 1 la therethrough. The dielectric stator means preferably comprises a polarized electret sheet material, for example, a sheet of Lexan obtained commercially from General Electric Company which is believed to be polycarbonate of the following composition: poly[2,2-bis(4- hydroxyphenyl) propane carbonate]. Each polycarbonate sheet 10, 11 may have a thickness of, for example, 50 mils and preferably is in the range of 1 mil to k inch. Such a sheet material of 50 mils thickness has the mechanical strength, stability and rigidity suitable for the electret electrode. The thickness of the conductive layer 10b, 1 lb on each electret may be, for example, 10 mils. A layer of electrically insulating material 12, 13, for example, a coating of approximately /2 mil thickness may be applied to the conductive layers 10b, 1 lb. For some constructions, a thin film electret could be laminated with a more rigid material, for example, a perforated metal plate to form the stator electrode of the desired mechanical strength and rigidity.

To form each stator 10, 11 a polycarbonate sheet may be, for example, sprayed, painted, or silk screened with a suitable conductive coating, for example, an adherent silver-loaded coating. The coated sheet may be cut to the desired size, and perforated in the desired pattern as represented schematically in FIG. 3. A conductive metal washer 14, which may be suitably cleaned, may be adhered with conductive epoxy to a projection 10c of the silver pattern surrounding an aperture in the stator to provide a suitable conductive connection between the silver coating and a terminal for making an electrical connection thereto to be described subsequently. After the conductive epoxy has set completely, the stator may be suitably cleaned. Another aperture 16 in the unsilvered region of the stator 10 is provided for means for making an electrical connection to the silver coating of the other stator 11 as will be described subsequently. The stators l and 11 preferably are of the same construction and silver pattern but the pattern 11b is positioned with its projection 11: extending in the opposite direction to the projection c as represented in broken line construction in FIG. 1.

The stator 10 has, for example, 90 apertures per square inch of 1/ 16 inch diameter in a pattern represented schematically in FIG. 3. That is, there are, for example, no apertures in the central region of, for example, one-half inch by one-half inch and no apertures in the edge region of, for example, /8 inch width of the silver pattern, to provide desired electromechanical conversion efficiency and sound transmission with little distortion in a frequency range of, for example, 50 hertz to 10 kilohertz. The dimensions of the silver region of the stator may, for example, be 3 inches by 3.5 inches and may, for example, have the shape of an ellipse. The aperture size, the number of apertures and their positions may be changed to change the efficiency, sound transmission and fidelity desired. Preferably the apertures are uniformly distributed as represented and are not clustered in a single region.

With a suitable coupling network between an electrical signal generator and earphones constructed in accordance with the invention, the earphones with apertures per square inch of l/16 inch diameter can develop an output of over decibels (relative to 0.0002 dyne/cm with a flat frequency response, that is, plus or minus three decibels, over a frequency range from 50 hertz to 10 kilohertz. Preliminary tests indicate that for apertures of 1/ 16 inch diameter approximately 50 apertures per square inch may provide maxiumum sound transmission with a flat frequency response, that is, plus or minus three decibels, over a frequency range from 50 hertz to 12 kilohertz.

Each stator may be polarized or charged, for example, by negative ion charge injection from an electrode at, for example, a negative 20 to 30 kilovolts across an air gap to the polycarbonate side of the stator while the silver side of the stator is grounded. The charge period may be, for example, /2 minute to 20 minutes. The stators preferably are charged simultaneously after assembly into a complete transducer in accordance with a preferred method of making the transducer to be described subsequently.

Referring again to FIGS. 1 and 2, the transducer also includes vibratory diaphragm means comprising a layer of conductive material 17a and 17b and means for supporting the diaphragm means between the stator means in spaced relation thereto.

Referring more particularly to FIGS. 4 and 5, the diaphragm means comprises, for example, two stretched polyethylene terephthalate films 18a, 18b available under the trade mark Mylar. Means for supporting the diaphragm means comprises, for example, cardboard frames 19, 20, 21 which are adhesively bonded in position in the transducer. I

At least one Mylar film of the diaphragm has a conductive layer thereon formed by, for example, a vacuum deposited aluminum coating. Preferably both films are conductively coated because better electronic and mechanical balance can be achieved in push-pull operations. The films are preferably so arranged that their insulating sides face their respective stators and their conductive coatings 17a, 17b face each other. Each of the Mylar films may have a thickness of, for example, one-half mil and each of the conductive coatings on the films a thickness of, for example, a few thousand Angstroms. The cardboard spacer 20 preferably has a conductive metalized surface and has a thickness of, for example, 10 mils and is inserted between the conductive surfaces and 17b of the diaphragm films to maintain them at the same electrical potential and to prevent the films from rubbing against each other at high frequency. The cardboard frames 19 and 21 may each have a thickness of, for example, 15 mils to provide an air gap of, for example, 15 mils thickness or distance between each diaphragm and the corre sponding stator. The dielectric constant of the polycarbonate sheet material is 3.02. Thus, the thickness of the electret sheet material of each stator 10, 11 preferably is greater than the thickness of the air gap and preferably is approximately equal to the product of the dielectric constant of the electret sheet material times the thickness of the air gap to provide maximum vibrational force on the diaphragm in response to an applied alternating-current voltage signal. To accomplish this end, with an air gap thickness of 15 mils, the thickness of the electret sheet material of each stator 10, 11 exd Kd,

where is the net surface charge density, d is the thickness of the electret, d: is the thickness of the air gap between the electret stator member and the conductive layer of the diaphragm, K is the dielectric constant of the electret and e is the permittivity of air.

From the above-mentioned article, it can be shown that the capacitance C between the electret stator member and the diaphragm may be expressed:

where a is the area of the surface of the electret stator member cooperative with the diaphragm.

Then the surface charge Q induced on the diaphragm, due to the alternating current signal between the electret stator member and the diaphragm, may be expressed:

QCV

where V is the alternating-current voltage signal applied between the stator electrode and the diaphragm.

According to Coulombs law, the vibrational force F on the diaphragm is proportional to the product of the direct-current field strength E and the surface charge 0 induced on the diaphragm due to the alternatingcurrent signal and may be expressed:

F Q z F CVE Substituting equations (l) and (2) in equation (5) --Vo' a d,

Differentiating equation (5a) with respect to K, the vibrational force F is a maximum when d K d2 In manufacturing the diaphragm assembly, stretched metalized film 170, 18a may be applied adhesively to a cardboard frame such as frame 19 represented in FIG. 5 by, for example, contact cement. The frame 21 may be similar to the frame 19 and a stretched metalized film 17b, 18b may be applied thereto in a similar manner. A cardboard spacer 20, such as represented in FIG. 6, metalized over its entire surface area by, for example, a conductive epoxy coating, may then be positioned between the films and the components of the diaphragm assembly may be adhered together with contact cement with conductive contact being maintained between regions of the metalized spacer 20 and the conductive surfaces 17a, 17b of the diaphragm films.

After the cement has set, the diaphragm assembly may be placed in a pre-heated oven at a temperature of, for example, C for a period of, for example, 2 minutes, until the Mylar film heat shrinks and suitable diaphragm tension results. As represented in FIG. 6, the metalized spacer has a slot 200 of, for example, 50 mils width, to allow gas pressure stabilization in the diaphragm assembly of FIG. 4.

The diaphragm assembly may then be adhesively bonded to the stators 10, 11 as represented in FIGS. 1 and 2 preferably with the apertures of both stators aligned in order to yield a higher acoustic transmissivity. Referring also to FIG. 7, which is a fragmentary sectional view to an enlarged scale of the FIG. 1 transducer taken along line 7-7 of FIG. 1, an opening 10d in the stator 10 and similar openings 19d and 21d through the cardboard frames 19 and 21 are openings for means for connecting electrically to the center metalized spacer 20 and to the conductive layers 17a, l7b of the diaphragm means, comprising, for example, a suitable terminal or conductive solder lug 33a held in conductive contact with the spacer 20 by a screw 31a which extends through conductive washer 320, the spacer 20 and the other cardboard frame 21 and is fastened by a suitable washer 34 and nut 34a.

Referring now more particularly to FIG. 2 of the drawings, there is represented in the fragmentary sectional view, drawn to an enlarged scale, of the FIG. 1 transducer, means for connecting electrically to the stator surface 10a. For example, a screw 25 extends through a lock washer 26 and conductive solder lug 27 and through conductive washer l4 and silver projection 10c of the surface 100, through an insulating compressible washer 28 of, for example, rubber, through a lock washer 29 and nut 30 on the other side of the transducer. Electrical connection is thereby assured between the solder lug 27 and the silver coating 10b. A generally similar connection may be made to the silver coating 11b of the other stator, as also represented in FIG. 2. For example, a conductive screw 31 extends through conductive lock washer 32, conductive solder lug 33, conductive washer 34, insulating compressible washer 35, silver coating projection 11c, conductive washer 36 and nut 37. In the FIG. 1 transducer the means for individually connecting electrically to the conductive layers of the stator means and the diaphragm means comprises means for applying electrical signals to the conductive layers of the stator means and the diaphragm means. t

The preferred method of making an electromechanical transducer comprises assembling in spaced relation a pair of dielectric stator means each having a conductive layer thereon and each having apertures therethrough and thin vibratory, diaphragm means comprising a layer of conductive material supported between the stator means. The method comprises electrostatically polarizing the assembled stator means by applying a high potential difference from the conductive layer of each stator member to the conductive layer of the diaphragm means. Preferably the step of polarizing the assembled stator means includes the step of applying the same high potential difference from the conductive layer of each stator means to the conductive layer of the diaphragm means. This may be done by, for example, connecting the two terminals of the stators to ground and connecting the terminal of the diaphragm means to a negative potential of, for example, kilovolts or more for a period of at least a few seconds to a half hour or more. The polarized dielectric stator means 10, ll develop direct-current electric fields of the same polarity relative to the layer of conductive material 17a, 17b of the diaphragm means.

By making the transducer in accordance with the preferred method just described, the operations during construction are simplified and there is a reduction of high voltage hazard during the polarizing or charging of the stators. Also, no handling of the electrets as individual components is necessary after charging and any adverse effect on the surface charge of the electrets, such as surface contamination by finger prints, is prevented. Transducers utilizing electret stators polarized by this method have higher efficiency and lower distortion than transducers utilizing stators polarized prior to assembly in the transducer by the previously-described method because the potential differences between the center diaphragm and the conductive coatings of the electret stators due to the surface charges of the elec tret stators are approximately equal after the charging process. This is important for distortion-free operation of push-pull type transducers. Also, the best electret stators are formed while an air space is maintained between one of the charging electrodes and the electret material. When the electret stators are charged after transducer assembly, air spaces are provided between the diaphragms and the electrodes comprising the silver coatings of the stators.

As compared with prior electrostatic earphones,

transducers constructed in accordance with the inven-' tion utilizing electrets charged prior to assembly in the transducers in the manner described previously also have high conversion efficiency because of the high equivalent direct current bias voltage of, for example, one kilovolt or more, and the transducers have low distortion because of push-pull operation, but the method of charging the transducer electrets after assembly is preferred to obtain even higher efficiency and lower distortion.

In FIG. 8 of the drawings there is represented a circuit diagram, partly schematic, of two earphones each comprising a transducer constructed in accordance with the invention and suitable for push-pull operation, to be coupled to the output circuits of a stereophonic audio system. The resistors 40 may individually have values of, for example, 10 megohms and are connected to the terminals of the stators 10, 11 and the diaphragm means 17a, 17b, 18a, 18b of each transducer as represented. The resistors may physically be positioned at the rear side of each transducer. The resistors 40 may not be necessary for some constructions and may be eliminated. Suitable transformer windings 41, 42, individually coupled to the left and right channels of a stereophonic audio source (not shown), supply audiofrequency electrical signals which cause push-pull operation of each of the transducers.

Loudspeakers comprising transducers in accordance with the invention can be constructed by combining a group of transducers to get more radiation area. The transducers in accordance with the invention can also be constructed as, for example, push-pull microphones and vibration pick-ups.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. The method of making an electromechanical transducer comprising assembling in spaced relation a pair of electret stator means each having a conductive member thereon and each having apertures therethrough, and thin, vibratory diaphragm means comprising a pair of layers of conductive material individually on a pair of films of plastic material supported between said stator means with each of said plastic films being between the conductive layer thereon and the stator means adjacent thereto, and electrostatically polarizing the assembled stator means by applying a high potential difference from said conductive member of each stator means to said conductive layers of said diaphragm means.

2. The method in accordance with claim 1 in which the step of polarizing the assembled stator means includes the step of applying the same high potential difference from said conductive member of each stator means to said conductive layer of said diaphragm 

1. The method of making an electromechanical transducer comprising assembling in spaced relation a pair of electret stator means each having a conductive member thereon and each having apertures therethrough, and thin, vibratory diaphragm means comprising a pair of layers of conductive material individually on a pair of films of plastic material supported between said stator means with each of said plastic films being between the conductive layer thereon and the stator means adjacent thereto, and electrostatically polarizing the assembled stator means by applying a high potential difference from said conductive member of each stator means to said conductive layers of said diaphragm means.
 2. The method in accordance with claim 1 in which the step of polarizing the assembled stator means includes the step of applying the same high potential difference from said conductive member of each stator means to said conductive layer of said diaphragm means. 